lvm2: don't overrwrite EXTRA_OECONF

EXTRA_OECONF has been assigned before ‘EXTRA_OECONF = "--with-usrlibdir=${libdir}"’.
collaps the 2 lines into a single EXTRA_OECONF.

Signed-off-by: Yao Xinpan <yaoxp@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>